The present invention provides a methodology for farfield analysis of noise sources (FANS) that represents a new concept for remote diagnostics of noise sources and for revealing the distribution of strengths of a sound source that is otherwise hard to obtain due either to limitations on the measurement devices or to inaccessibility of measurement transducers. For example, in analyzing the distribution of vehicle pass-by noise source or that of turbulence over a car body inside a wind tunnel, or the strength of a hot jet engine or that of wind turbine, it will be nearly impossible to place an array of microphones next to the source. This is because the presence of any measurement devices near a source may either completely alter the acoustic pressure field that is being measured such as in diagnosing sources of vehicle pass-by noise, or the condition and environment are so hostile that it can completely damage a transducer such as in the case of a hot jet engine, or the only way to measure the acoustic pressure is to do it in a far field such as in the case of a wind turbine.
Under these conditions, the well-known nearfield acoustic holography (NAH) is not an option, even though it is very accurate and can provide all the encompassing acoustic quantities generated by a sound source. It is emphasized that in these circumstances, it will not be possible to obtain precise measurements of an acoustic pressure field and only an estimate of the acoustic pressure can be made. Nevertheless, in practice such adversary situations are often encountered in which the acoustic pressure at the location where microphones cannot be placed is desired. There are other cases in which a quick estimate of the strength of a noise source is required and for which NAH may not be the method of choice because it is considered time consuming and costly. So it will be highly desirable to have a methodology that can provide a quick estimate of the strength of a noise source in a far field.
Presently, there are no reliable methods available for diagnosing source strengths on an arbitrary source surface based on the far-field acoustic pressure measurements. Microphone only provides the sound pressure level (SPL) value or spectrum at a measurement location. NAH can correlate the near-field acoustic pressure measurements to the acoustic field on a source surface. However, when measurements are taken in a far field, the near-field effects are lost and the ratio of a measurement aperture to the standoff distance is substantially reduced in comparison to that in near-field measurements. As a result, the input data are severely inadequate and the problem of reconstruction becomes so highly ill posed that the results thus obtained are meaningless even if regularization is utilized.
To address the need for far-field source identification, some rudimentary methods were proposed, including the beam forming technology developed by Acoustic Camera and Bruel and Kjær. Beam forming is a spatial filter that operates on the output of an array of sensors in order to enhance the amplitude of a coherent wavefront relative to background noise and directional interference. In applying beaming forming to noise source identification, the acoustic pressure is measured by an array of 30 to 60 microphones in the far field. The phases of these measured acoustic pressures are adjusted so that a maximum pressure lobe is identified. The orientation of this pressure lobe indicates where the sound wave is coming from.
The original beamforming makes use of a superposition of plane waves to localize sound source within an angular sector. Since the plane wave has constant amplitude, this approach can only tell the direction of sound wave propagation, but not the actual location of the source. An improved beamforming utilizes a superposition of spherical waves. The phase delay calculations of the spherical waves allow for focusing on a source location. However, the acoustic images are displayed on a plane parallel to the microphone array, so it is two dimensional. Moreover, there is no way of identifying the source strengths on the surface of an arbitrarily shaped object.